The present invention is related to wireless communication, and in particular to a method and apparatus for detecting, reporting, and either avoiding or mitigating hidden nodes in a wireless data communication network.
Wireless communication networks are already in common use and will become more so as new standards emerge and systems become more widely deployed. One application of wireless networks is data networks such as local area networks. The IEEE 802.11 is a popular standard for wireless local area networks (WLANs). Variations include the IEEE 802.11b standard for communications in the 2.4 GHz range for transmission rates to about 11 Mbps (megabits per second), now popularly called “WiFi”, the IEEE 802.11a standard for communications in the 5 GHz range for transmission rates to about 54 Mbps or more, now popularly called “WiFi-5,” and the IEEE 802.11g proposed standard for communications in the 2.4 GHz range at up to 54 Mbps.
The description herein is in terms of networks that conform to the 802.11 standard. The invention, however, is not restricted to such wireless communication networks.
A Basic Service Set (BSS) consists of two or more wireless nodes, also called stations (STAs), which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer basis, sharing a given cell coverage area. This type of network is often formed on a temporary basis, and is commonly referred to as an ad hoc network or Independent Basic Service Set (IBSS).
In most instances, the BSS contains a node that acts as an Access Point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is analogous to a base station used in cellular phone networks. An AP can also work as a repeater. When an AP is present, the stations do not normally communicate on a peer-to-peer basis. Usually all communications between stations or between a station and a wired network client go through the AP. APs are typically not mobile and typically form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode. The term BSS shall refer to such an infrastructure network, while the term IBSS shall be used for an ad hoc configuration.
A station or node is a hidden node to a second node if that node—the first node—cannot be “heard” at the second node, and transmissions from the first node can collide with those from the second node at some third node. The first node may be hidden because of some physical barrier in the path to the second node, for example a thick wall, or because the second node is far away.
The existence of one or more hidden nodes in a wireless network may reduce the performance in the network. This is particularly so for large deployments that include many nodes. Consider for example a network conforming to the IEEE 802.11 standard. The MAC protocol for the IEEE 802.11 standard uses a carrier sense multiple access with collision avoidance (CSMA/CA) protocol to provide basic access to the medium and a virtual carrier sense mechanism to improve the basic access mechanism. The CSMA/CA protocol relies on the energy from every node to be received at every other node, while the virtual carrier sense mechanism relies on MAC frames from every node to be received at every other node. The presence of one or more hidden nodes means energy and MAC frames are not received at every other node. This increases the probability of collisions, which in turn wastes bandwidth. This waste becomes larger the more nodes there are in the network and the more hidden nodes that may exist in the network. See, for example, Sumit Khurana, Anurag Kahol and Anura P. Jayasumana, “Effect of Hidden Terminals on the Performance of IEEE 802.11 MAC Protocol,” Proceedings of the 23rd. IEEE Annual Conference on Local Computer Networks, IEEE, 1998.
Thus, there is a need in the art for a method to detect and report the presence of hidden nodes. There also is a need in the art for a method to mitigate or avoid hidden nodes.
Known methods for dealing with hidden nodes in the IEEE 802.11 standard provide only for mitigating the effects of hidden nodes. For example, the IEEE 802.11 MAC specification provides for optionally using a short frame called a “request to transmit” (RTS) and waiting for a short response frame called a “clear to send” (CTS) for the purpose of mitigating the effects of hidden nodes. If every node in the network uses RTS/CTS, collisions are guaranteed to occur only for short RTS frames and not potentially long DATA frames. When RTS/CTS is used, RTS/CTS thresholds must be set on both APs and STAs. Thus, a user must manually detect or deduce the presence of hidden node(s) and if desired, manually turn on the hidden node mitigation technique, for example by enforcing the use of RTS/CTS by lowering the RTS/CTS trigger threshold. However, enforcing the use of RTS/CTS may lower the overall system throughout because of the overhead for the extra frame exchange. Thus, it would be advantageous to avoid the hidden node problem altogether. Furthermore, the IEEE 802.11 standard provides no mechanism for automatically detecting and reporting the presence of the hidden nodes, and there clearly would be an advantage in so automatically detecting and reporting the presence of the hidden nodes in a network conforming to the IEEE 802.11 standard. Once the hidden nodes are reported, the appropriate mitigating or avoiding action may be taken.